Researchers at the National Institute of Standards and Technology (NIST) announced that they have dramatically improved the efficiency and laser power output of a range of chip-level device devices in two new studies. These devices can produce different colours of laser light while using the same input laser source -- providing a new way of producing laser light on integrated microchips.
Applied to the next generation of quantum devices
Many quantum technologies, including tiny optical atomic clocks and future quantum computers, will require simultaneous access to multiple, widely varying laser wavelengths in a small region of space. For example, some of the leading atom-based quantum computing designs require up to six different laser wavelengths for all the steps required, including preparing the atoms, cooling them, reading their energy states, and performing quantum logic operations.
To generate multiple wavelengths on a chip, NIST researcher Kartik Srinivasan and colleagues have spent the past few years studying nonlinear optical devices, such as those made from silicon nitride, that have the property that the wavelength of the laser entering the device may be different from the wavelength of the laser exiting.
For their experiment, the researchers employed a process known as third-order optical parametric oscillation (OPO), in which incoming light is subsequently converted into two different wavelengths, each corresponding to two different frequencies. For example, a near-infrared laser incident on a material is converted into a visible laser with a shorter wavelength and an infrared laser with a longer wavelength (at a lower frequency).
Previously, the team demonstrated that this conversion process, known as "optically parametric oscillation," can be achieved in silicon nitride microresonators, a ring device small enough to be manufactured on a chip.
The light runs through the ring about 5,000 times, creating an intensity high enough for the silicon nitride to convert it into two different frequencies. The two wavelengths are then coupled into a straight rectangular channel, also made of silicon nitride -- located next to each other in the ring -- that acts as a transmission line or waveguide and transmits light to where it is needed.
Wide wavelength range
Previously, the specific wavelength generated was determined by the size of the microresonator and the wavelength of the input laser. Because many microresonators of slightly different sizes are created during the manufacturing process, the technique provides a wide range of outputs on a single chip, all using the same input laser.
However, Srinivasan and his colleagues, including researchers from the Joint Quantum Institute (JQI), a collaboration between the NIST Institute and the University of Maryland, all found that the process is very inefficient - less than 0.1 percent of the input laser is converted to either of the two output wavelengths in the waveguide. The team observed that most of the inefficiencies were due to poor coupling between the ring and the waveguide.
In the first study, published in APL Photonics, Srinivasan and his NIST/JQI collaborators redesigned the straight waveguide so that it was U-shaped and wrapped around part of the ring. With this modification, the researchers were able to convert about 15 percent of the incident light into the desired output, more than 150 times more than in their earlier experiments.
Power milestone
In addition, the converted light has more than 1 milliwatt of power over a wide wavelength range from visible to near infrared. Generating 1 milliwatt of power is a milestone, Srinivasan said, because that level is sufficient for many applications. For example, it can cause a tiny laser to excite electrons to jump or jump from one particular energy level to another within an atom.
In addition, the milliwatt power level is sufficient to stabilize the laser. Some atoms have very stable transition energies and are not sensitive to environmental effects, and their study provides a good reference by which to compare and correct laser frequencies and ultimately improve their noise characteristics.
In the second study (published in Nature Communications), Srinivasan and his colleagues, led by Edgar Perez, further improved the technology's power output and efficiency. By increasing the coupling between the ring and waveguide and suppressing effects that could interfere with color conversion, the team increased the output laser power to up to 20 milliwatts and converted up to 29 percent of the incident laser to the output wavelength.
Source: OFweek
